As is well known, elevator cars typically employ doors which open and close to allow passenger transfer between the car and the floor landings. Modern, high-speed elevators can typically travel at speeds on the order of 200 meters per minute, and therefore travel from one landing to the next, at full speed without stopping, in about 1 second. For each stop, the elevator must decelerate and reaccelerate, and open its door long enough for passenger transfer. The typical time for a modern elevator landing stop is on the order of 13 or 14 seconds, and this time accumulates quite rapidly in elevators serving many floors. Therefore, an elevator design goal is to open and close the doors in the least possible time. However, passenger confidence requires that the doors work fairly smoothly, so as not to portray an image of elevator unreliability. Also, door velocity is limited, to limit the kinetic energy of the door to a reasonably safe level, particularly when closing, so that passengers will not be injured by the door. Also, the door must be decelerated to a stop and reaccelerated, within a requisite distance, as required by safety codes, whenever a passenger activates door safety device during closing.
In elevators known to the art, the most common type of door operator employs analog door motor control, the voltage supplied to the motor being controlled by varying resistances in the circuit in accordance with door position, utilizing cam switches spaced along the travel path of the door, relays, potentiometers and the like. These devices, being basically mechanical in nature, are subject to wear and require frequent adjustment, causing a major portion of the maintenance cost thereof. Additionally, optimal operation can never be achieved with such systems since a perfect adjustment is not possible, and adjustments to perfect one parameter frequently detract from another parameter. Because such devices are imperfect and are adjusted on a door by door basis, the characteristics are not capable of being theoretically defined in advance, and diagnostics of problems are extremely difficult.